Force of Will is a mana accelerant. You are trading two cards and zero mana for one card and however much mana your opponent spent on their spell. Alternatively, you can think of it as spending a card generate two blue mana with which you cast Counterspell.

By comparison, Unmask generates one black mana to cast Thoughtseize. You're not generating as much mana and you're not getting as good an effect. You are trading two cards and zero mana for one card and zero mana. That's not a good baseline rate. You need to be working pretty hard or have a reason for discarding (i.e. Dredge) in order to make it worth it.

I don't think blue control runs enough black cards to reliably cast Unmask. And the free aspect isn't worth a card imo. Also, I'm no expert on storm, but I think they wouldn't want to pitch resources to cast Thoughtseize. If I had to pitch a discard spell, I would rather just cast it and pitching a bomb or ritual sounds pretty bad (it's almost like you cast Thoughtseize on both players :) ).

None of these applies to dredge, as you may not even have access to Thoughtseize mana and you don't care about resources - one dredger with bazaar is all you need to start.

Wappla hit on something important.
Proactive discard effects like thoughtseize and duress are tempo negative. Don't take it from me either, Reid Duke wrote an entire article on thoughtseize and said "strictly speaking, casting thoughtseize puts you a little behind". This is because you've spent Mana and a card to stay at parity (or more accurately the same amount of card advantage/disadvantage).
This is why thoughtseize was often bad in aggro decks but great in midrange or control decks. It's often times worth the slight disadvantage in terms of Mana spent to strip an important card away, but when you're relying on tempo and speed its often not so great.
The only deck I could think of that would really want an unmask would be storm, but the loss of a card would be terribly damaging to that deck's strategy. In dredge you're relying on cards you can cast with no Mana, especially ones you can cast from your graveyard. Unmask makes much more sense in that deck.

I should mention that there was a fringe deck in legacy called burning Reanimator that used to play unmask. The deck wanted to.set up broken plays on turn one and utilized lions eye diamond, so unmask was a good fit.
If you had a deck that was similar and good enough for vintage, maybe unmask would be OK.

i feel like for storm the combination of duress +therapy/probe is simply better. spending an extra black card like dark petition isn't awful but allot of the time you simply want to just cast the extra "threat" rather then try to beat counter magic. i could see unmask being a reasonable card against shops or eldrazi if you simply wanted to not die to infinite spheres and thot nots on turn 1 or 2.